1. Field of the Invention
The present invention relates to an optical printer whose printing head is provided with an array of light emitting elements each recording one pixel at a time on a photosensitive recording medium, wherein a grayscale image is reproduced by controlling the light emitting elements each individually. The present invention relates also to a method for driving the optical printer.
2. Background Arts
Many kinds of optical printers that are driven based on electronic image data to print an image on a sheet of self-developing type photo film, so-called an instant film unit have been known in the art. Also electronic still cameras integrated with such an optical printer have been on the market. An exemplary of optical printer is provided with a printing head that has an array of light emitting elements arranged along a main scan direction, and a color filter for obtaining red, green and blue light beams from white light beams projected by the light emitting elements. The optical printer selectively drives the light emitting elements to project color light beams toward the instant film sheet while moving in a sub scan direction perpendicular to the main scan direction, whereby an image is recorded line after line on the instant film unit. The optical printer also controls exposure amounts by changing lighting times of the individual light emitting elements, i.e. exposure times for each pixel, to reproduce gradations. In practice, a unit exposure time is predetermined, and each light emitting element is driven for a multiple of the unit exposure time by being applied with a different number of drive pulses having a constant width.
FIG. 10 shows a characteristic curve 81 showing relationships between exposure amount of the instant film unit and coloring density of a positive image formed on an image receptive sheet of the instant film unit. As well-known in the art, a photosensitive sheet of the instant film unit is exposed to an optical image to form a latent image. Thereafter, a processing fluid is spread between the photosensitive sheet and the image receptive sheet, so the latent image is transferred to the image receptive sheet and developed as a positive image. Accordingly, the positive image gets the higher coloring density, the smaller the exposure amount of the photosensitive sheet. In other words, the larger the exposure amount of the photosensitive sheet, the positive image gets the lighter tones.
FIG. 11 shows a relationship between gradations of the positive image and exposure amount of the instant film unit in a low density range, whereas FIG. 12 shows a relationship between gradations of the positive image and exposure amount of the instant film unit in a high density range. As seen from these graphs, it takes a large increment or decrement in the exposure amount to change the coloring density by one tonal step in the low density range. On the contrary, in the high density range, the density changes by one tonal step with a very small difference in the exposure amount, so a fine control of the exposure amount is necessary for reproducing gradations.
Since the lighting time of each light emitting element is controlled by the unit exposure time in the conventional optical printer while maintaining the luminance constant, it is necessary to define the unit exposure time to be short enough for permitting a fine control of the exposure time, in order to reproduce gradations even in the high density range. On the contrary, the exposure time must be changed by a remarkably larger amount to change the coloring density by one tonal step in the low density range than in the high density range. This is apparent from FIGS. 11 and 12, wherein T1max is an increment or difference in the exposure time necessary for changing the density from the second lowest tonal level to the lowest tonal level, whereas T1min is an increment in the exposure time necessary for changing the density from the highest tonal level to the second highest tonal level, assuming that the density of the highest tonal level, i.e. black, is obtained by not exposing the instant film unit to the light.
For these reasons, in a conventional optical printer, to reproduce the image in 256 gradations, i.e. in tonal levels of “0” to “255”, the unit exposure time is defined to be T/1023, provided that T represents a longest exposure time necessary for coloring the image receptive sheet at a lowest density, i.e. the tonal level “0” that is approximately equal to white. In other words, the exposure time is controlled with a precision of T/1023.
Although the conventional method achieves the fine control of the exposure time, a remarkably larger number of drive pulses must be applied for recording one pixel in the low density range, so that a very high driving frequency is needed in order to cut the longest necessary exposure time T per one pixel and thus speed up the printing. Furthermore, because 10-bit control data is needed for controlling the exposure time with the precision of T/1023, it takes a certain time to process and transfer 10-bit control data, so that a very high processing speed is required for cutting down the longest exposure time T. Indeed the exposure time can be shortened by raising luminance from the printing head, because the data processing speed is limited, if the longest exposure time T does not have a certain length, the unit exposure time T/1023 would become too short to drive the light emitting elements. For these reasons, it has been hard to speed up the printing time in the conventional optical printer.